EP0328169B1 - Process for manufacturing sodium perborate monohydrate, partially crystallized sodium perborate monohydrate and textile washing powders - Google Patents

Abstract

Process for producing sodium perborate monohydrate by dehydration of sodium perborate tetrahydrate using a current of air which is initially damp in a first phase of dehydration then dry in a second phase of dehydration. The sodium perborate monohydrate obtained can be used in washing powders for textiles. No diagrams.

Description

The invention relates to a process for the production of sodium perborate monohydrate by dehydration of sodium perborate tetrahydrate.

Sodium perborate monohydrate, with the crude formula NaB0 ₃ .H ₂ 0, finds an important outlet in the preparation of washing powders, due to its high content of active oxygen and its high rate of dissolution in water.

A technique commonly used to obtain sodium perborate monohydrate consists in dehydrating sodium perborate tetrahydrate (of crude formula NaB0 ₃ .4 H ₂ 0) in a fluidized bed, by means of a current of hot air. In known processes in accordance with this technique, operating conditions have generally been sought which make it possible to obtain sodium perborate monohydrate of high hardness, so that it has a high resistance to attrition. For this purpose, in the process described in patent BE-A-718160 (SOLVAY & Cie), ambient air is used, preheated to a temperature of at least 180 _° C, preferably between 180 and 210 ° vs. The use of such high temperatures is however expensive and it implies taking precautions taking into account the oxidizing nature of sodium perborate.

In patent BE-A-807724 (PEROXID-CHEMIE GmbH), it is recommended to use hot and humid air, the temperature and humidity of which are adjusted, at the entrance of the bed, so that the temperature of air, at the exit of the bed, that is to say at least 60 ° C., so as to cause a surface fusion of the sodium perborate. The sodium perborate monohydrate obtained by this known process has the particularity that its active oxygen content decreases rapidly when it is exposed to ambient atmospheric air. This feature constitutes a serious drawback, because it involves taking special measures to store or handle sodium perborate monohydrate and it also leads to rapid degradation of the detergent properties of the washing powders.

The invention aims to remedy the drawbacks of known methods, by providing a new method which makes it possible to obtain, by dehydration of sodium perborate tetrahydrate, sodium perborate monohydrate having simultaneously a high resistance to attrition and good stability in regarding its active oxygen content.

The invention therefore relates to a process for the production of sodium perborate monohydrate by dehydration of a bed of sodium perborate tetrahydrate in a stream of air. According to the invention, an air current is used which is first humid in a first phase of dehydration, then dry in a second phase of dehydration.

In the process according to the invention, the air used at the inlet of the sodium perborate tetrahydrate bed is hot air, the temperature of which is generally greater than 60 _° C. and, preferably, at least equal to 80 ° C; it is desirable that its temperature does not exceed 180 ° C. Temperatures between 90 and 150 _° C are the most suitable.

The air flow rate to be used depends on various parameters, among which are in particular the particle size of the sodium perborate tetrahydrate, the volume of the bed and the area of its section as well as the specific mass sought for the sodium perborate monohydrate ; it must be adjusted to ensure a stable fluidized bed for the duration of the dehydration. In practice, the air flow rate, at the inlet of the bed, can for example be fixed between 0.5 and 30 Nm ³ per hour and per kg of sodium perborate tetrahydrate, the values between 1 and 20 Nm ³ being the most adequate in the majority of cases.

Dry air is by definition anhydrous air or, more generally, air taken from the atmosphere, the partial pressure of water vapor does not exceed the saturation vapor pressure under normal conditions of pressure and temperature; it is therefore most often ambient air having a relative humidity of between 40 and 100% under normal conditions of atmospheric pressure and temperature.

The moist air used in the first phase of dehydration is air whose absolute humidity is greater than that of the dry air used in the second phase of dehydration. According to the invention, humid air can advantageously be used, the moisture content of which is higher than that corresponding to the saturated vapor pressure of the ambient air under normal conditions of atmospheric pressure and temperature. However, the total amount of water in the humid air at the inlet of the bed should be less than that corresponding to the saturation vapor pressure of the air at the temperature it has at the outlet of the bed. The optimum value of the total amount of water in the humid air used in the first phase of dehydration therefore depends on other operating conditions such as the temperature and the flow of humid air at the inlet of the bed. and the temperature of the bed; it can be easily determined in each particular case by routine laboratory work.

In the process according to the invention, the relative importance of each phase of dehydration depends on the quality of the sodium perborate tetrahydrate used and on the characteristics sought for the sodium perborate monohydrate (in particular its hardness and its stability as regards concerns its active oxygen content). In practice, it is recommended that the duration of the first phase of dehydration does not exceed 50% of the total duration of dehydration. It is desirable that the duration of the first phase is at least equal to 2% of the total duration of the dehydration, the values between 5 and 40% being preferred.

In a particular embodiment of the method according to the invention, the humidity content of the air used in each phase of the dehydration is adjusted so that the relative humidity of the air, in thermal equilibrium with the phase solid at the end of the bed, ie between 15 and 80% in the first phase and between 10 and 60% in the second phase. Preferred values for the relative humidity of the air leaving the bed are those between 20 and 70% in the first phase and between 15 and 50% in the second phase, the relative humidity of the air in the first phase being greater than that of air in the second phase.

In a particularly advantageous embodiment of the process according to the invention, the temperature of the air which is introduced into the bed is identical in the two phases of dehydration, so that the temperature of the bed in the first phase of dehydration is greater than that which it has in the second phase.

In this embodiment of the method according to the invention, the difference between the temperatures of the bed in the first phase and in the second phase of the dehydration is between 1 and 10 ° C. Values between 2 and 7 _° C for said difference are preferred. For this purpose, one can for example adjust the temperature and the humidity of the air so that the temperature of the bed is fixed between 45 and 85 _° C during the first phase of the dehydration and between 40 and 80 _° C during the second phase, the preferred temperatures being those between 50 and 70 ° C for the first phase and between 45 and 65 ° C for the second phase, the temperature of the bed in the first phase being higher than that which it has in the second phase . These temperatures can easily be obtained in the bed by fixing the air temperature, at the inlet of the bed, between 80 and 180 ° C, the temperatures between 90 and 150 _° C being preferred. According to a particular variant of this embodiment of the method according to the invention, it is preferred to select, for the first phase of dehydration, a bed temperature which is lower than that usually called "melting temperature" of sodium perborate, which generally does not exceed 65 _° C. In this variant of the process, the preferred temperatures for the bed are those between 52 and 62 ° C in the first phase of dehydration and between 50 and 60 _° C in the second phase.

At the end of the second phase of dehydration, an increase in the temperature of the bed is generally observed, which is an indication of the end of the dehydration of sodium perborate from the tetrahydrate stage to the monohydrate stage.

In the process according to the invention, it is generally advantageous to precede the first phase of dehydration by a step of bringing the sodium perborate tetrahydrate to temperature.

All other things being equal, the properties of the sodium perborate monohydrate obtained at the end of the process according to the invention will depend on those of the sodium perborate tetrahydrate used. In particular, under identical operating conditions, the attrition resistance of sodium perborate monohydrate is generally all the better as that of sodium perborate tetrahydrate is good. It is therefore advantageous to use sodium perborate tetrahydrate having a high attrition resistance, characterized for example by an index not exceeding 4.5% according to ISO standard 5937. It is especially recommended to use sodium perborate tetrahydrate whose attrition resistance index according to ISO 5937 does not exceed 3%, preferably 2.5%.

The process according to the invention can be carried out discontinuously in a single fluidized bed reactor which is successively supplied with moist air and with dry air. Alternatively, the process can be carried out continuously in a succession of two fluidized bed reactors, one of which is supplied with moist air and the other with dry air. It is also possible to use a number of reactors greater than two, which are supplied with air, the moisture content of which decreases from one reactor to the next. In this last variant of the process according to the invention, it is also possible to adjust the air flow rate so that it goes decreasing from one reactor to the next.

In the process according to the invention, the sodium perborate monohydrate produced during the first phase of dehydration has a preponderance of amorphous material, while that formed during the second phase of dehydration is mainly in the crystalline state. The sodium perborate obtained by means of the process according to the invention therefore comprises a mixture of crystalline material and of amorphous material which have been demonstrated by the X-ray diffraction method. It combines a high resistance to attrition and good stability characterized by a moderate speed of its loss of active oxygen over time. Without wishing to be bound by a theoretical explanation, the applicant believes that these superior properties of sodium perborate monohydrate would be attributable to the morphological structure of its grains, which would have a crystalline nucleus in an envelope in an amorphous state. The aforementioned good stability of sodium perborate monohydrate would therefore be the result of the crystalline nature of its core, while the higher hardness of its amorphous envelope would give it its high resistance to attrition. Furthermore, starting from sodium perborate tetrahydrate of equal quality, the process according to the invention makes it possible to obtain sodium perborate monohydrate whose specific surface is greater than that obtained with the known process described in document BE-A- 807724.

The invention therefore also relates to sodium perborate monohydrate, obtainable by the process according to the invention, which is in the partially crystallized state and which has an amorphous content content at most equal to 60% by weight, a attrition wear rate equal to a maximum of 10% and a stability corresponding to a weight loss of active oxygen less than 60% of its original content, after 50 days of storage in ambient air at 55 _° vs.

In the above definition of sodium perborate monohydrate according to the invention, the attrition rate is defined according to ISO standard 5937.

The invention relates in particular to sodium perborate monohydrate which contains between 20 and 60% by weight of amorphous material, for example between 35 and 60%, and between 150 and 160 g of active oxygen per kg and which has a rate of '' attrition wear between 2 and 8% and a stability corresponding to a weight loss of active oxygen between 20 and 55% of its original content, after 50 days of storage in air at 55 _° C .

A specially advantageous variety of sodium perborate monohydrate according to the invention has a specific surface of less than 10 m ² / g, for example between 4 and 8 m ² / g.

Thanks to its high attrition resistance and its good stability with regard to its active oxygen content, the sodium perborate monohydrate according to the invention finds an interesting outlet in the manufacture of powders for washing textiles.

The invention therefore also relates to washing powders for textiles, which contain sodium perborate monohydrate according to the invention.

The advantage of the invention will be highlighted on reading the application examples which are described below. In these examples, the expression "dry air" designates air taken from the atmosphere, in accordance with the definition which has been given above. The attrition resistance is defined according to ISO standard 5937. The active oxygen content was determined by titrimetry using potassium permanganate and is expressed in g of 0 ₂ per kg of material.

1. First series of examples (examples 1 to 4)

In Examples 1 to 4 which follow, sodium perborate tetrahydrate having the following characteristics was used:

2 kg of sodium perborate tetrahydrate were loaded into a fluidized bed dryer which was supplied with ambient air containing 14 g of steam per kg, preheated to 100 _° C., at a rate of flow rate of 16.5 Nm ³ / h.

Examples 1 and 2 relate to two tests which were carried out under conditions in accordance with the invention. Examples 3 and 4 relate to reference tests which were carried out under conditions which are not in accordance with the method according to the invention.

Example 1 (according to the invention)

• Etape n ° 1 : mise à température du perborate de sodium tétrahydraté;
• Etape n _° 2 : première phase de la déshydratation au moyen d'air humide;
• Etape n ° 3 : deuxième phase de la déshydratation au moyen d'air sec.
  • Pendant l'étape n ° 1, qui a duré 7 minutes, le lit de perborate de sodium a été alimenté avec de l'air ambiant préchauffé à 100°C, comme exposé plus haut.
  • Pendant l'étape n ° 2, qui a duré 43 minutes, on a introduit de la vapeur d'eau dans l'air, en amont du lit.
  • Pendant l'étape n ° 3, qui a duré 50 minutes, on a cessé l'alimentation en vapeur d'eau, de sorte que l'air admis dans le lit était de l'air ambiant tel quel.

The operation of the bed consisted of the following three successive stages:

• Step 1: warming up the sodium perborate tetrahydrate;
• Step _2: first phase of dehydration by means of moist air;
• Step 3: second phase of dehydration using dry air.
  • During step 1, which lasted 7 minutes, the sodium perborate bed was supplied with ambient air preheated to 100 ° C, as described above.
  • During stage 2, which lasted 43 minutes, steam was introduced into the air, upstream of the bed.
  • During stage 3, which lasted 50 minutes, the steam supply was stopped, so that the air admitted into the bed was ambient air as it was.

At the end of step _3, the bed temperature increased rapidly, indicating the end of the conversion of sodium perborate tetrahydrate in the form of monohydrate.

The operating conditions are mentioned below:

Example 2 (according to the invention)

The conditions of Example 1 were reproduced, except as regards the durations of steps nos. 2 and 3, which were respectively 28 minutes (step # 2) and 70 minutes (step # 3). The bed temperature is set at approximately the same values as those of Example _No. 1.

Example 3 (for reference)

Was repeated the conditions of Example 1, with the sole exception that one has failed to introduce the water vapor in the air during step ₂ (first phase of dehydration). The bed was therefore continuously supplied with dry air for 100 minutes. During dehydration, the bed temperature stabilized at 53 ° C and the relative humidity of the air leaving the bed was fixed at 28%. At the end of the process (which lasted 100 minutes), the temperature of the bed increased to 66.7 ° C.

Example 4 (for reference)

The conditions of the test of Example 1 were modified, by introducing water vapor into the air, upstream from the bed, during the two phases of dehydration (Steps nos. 2 and 3). During dehydration, the bed temperature was established at 60.5 ° C and the relative humidity of the air leaving the bed was established at 45%. After 100 minutes of treatment, the bed temperature rose to 67.7 ° C.

The characteristics of the sodium perborate monohydrate obtained in each of Examples 1 to 4 which have just been described are reproduced in Table I.

In Table I, the attrition resistance was measured by the ISO 5937 method. The active oxygen content after 7, 25 and 50 days was determined after maintaining the sodium perborate monohydrate in contact with ambient air. at 55 ° C.

It is observed that the sodium perborate monohydrate obtained in Examples 1 and 2 (in accordance with the invention) present an optimum compromise with regard to the resistance to attrition and the loss of active oxygen.

He. Second series of examples (examples 5 to 7)

In these examples, sodium perborate tetrahydrate having the following characteristics was used:

2 kg of this sodium perborate tetrahydrate were loaded into the fluidized bed dryer, and the latter was supplied with ambient air containing 16.6 g of steam per kg, preheated to 118 ° C., at a rate of 13.2 Nm ³ / h.

Example 5 relates to a test which was carried out under conditions in accordance with the method according to the invention. Examples 6 and 7 relate to reference tests under conditions which are not in accordance with the process according to the invention.

Example 5 (according to the invention)

The operation of the bed consisted of three stages nos. 1, 2, 3 defined above in Example 1; these were executed under the following conditions:

Example 6 (reference)

Was repeated the conditions of Example 5, except that failed to introduce the water vapor in the air during the step _No. 2. The total duration of treatment in the reactor bed fluidized was also 105 minutes, during which the bed was therefore supplied with dry air exclusively. The bed temperature stabilized at 53.5 ° C and the relative humidity of the air leaving the bed stabilized at 28%. After dehydration, the bed temperature rose to 66 ° C.

Example 7 (for reference)

The conditions of the test of Example 5 were modified, using ambient air, into which an excess of water vapor was also introduced during the entire dehydration treatment which lasted 108 minutes. During dehydration, the temperature of the bed was established at 61.5 ° C and the relative humidity of the air leaving the bed was established at 40%. After dehydration, the bed temperature rose to 66 ° C.

The characteristics of the sodium perborate monohydrate obtained in each of Examples 5 to 7 are recorded in Table II.

A comparison of the results of Examples 5, 6 and 7 shows that the sodium perborate monohydrate of Example 5 (according to the invention) combines optimum characteristics of attrition resistance and stability with regard to the content of active oxygen.

In the tests which have just been described in examples 1 to 7, the specific surface was measured using the BET method based on the determination of the nitrogen adsorption and desorption isotherms (Journal of the American Chemical Society - Vol. 60 (1938) - S. Brunauer, PH Emmet, E. Teller: "Adsorption of Gases in Multimolecular Layers" - pages 309 to 319).

Claims (13)

1. Process for producing sodium perborate monohydrate by dehydration of a bed of sodium perborate tetrahydrate in a current of air, characterised in that a current of air is used which is initially damp in a first phase of dehydration, then dry in a second phase of dehydration, the absolute humidity of the air in the second phase being less than that of the first phase.

2. Process according to Claim 1, characterised in that ambient air having a relative humidity of between 40 and 100 % under the standard conditions of temperature and atmospheric pressure, is used as the dry air.

3. Process according to Claim 1 or 2, characterised in that the humidity content of the air used is regulated, so that the relative humidity of the air at the outlet from the bed is between 15 and 80 % in the first phase of dehydration and between 10 and 60 % in the second phase, the relative humidity of the air which is provided in the first phase being greater than that of the air which is provided in the second phase.

4. Process according to any one of Claims 1 to 3, characterised in that the duration of the first phase of dehydration is between 2 and 50 % of the total duration of dehydration.

5. Process according to any one of Claims 1 to 4, characterised in that the temperature of the air which is introduced into the bed is the same in the two phases of dehydration, so that the temperature of the bed in the first phase of dehydration is greater than its temperature in the second phase.

6. Process according to Claim 5, characterised in that the temperature and the humidity of the air are regulated at the inlet to the bed, so that the difference between the temperatures of the bed in the first phase and in the second phase of dehydration is fixed between 1 and 10°C.

7. Process according to either of Claims 5 or 6, characterised in that the temperature of the bed in the first phase of dehydration is at most equal to the melting temperature of sodium perborate.

8. Process according to Claim 7, characterised in that the temperature of the bed is between 52 and 62 _° C in the first phase of dehydration and between 50 and 60 _° C in the second phase.

9. Process according to any one of Claims 1 to 8, characterised in that sodium perborate tetrahydrate is used having an index of resistance to attrition, according to the standard ISO 5937, not exceeding 4.5 %.

10. Partially crystallised sodium perborate monohydrate, which has a content of amorphous material of at most 60 % by weight, a degree of consumption by attrition, according to the standard ISO 5937, of at most 10 % and a stability corresponding to a weight loss of active oxygen less than 60 % of its original content, after 50 days of storage in ambient air at 55 _° C.

11. Sodium perborate monohydrate according to Claim 10, characterised in that its content of amorphous material is between 20 and 60 %, its original content of active oxygen is between 150 and 160 g per kg, its degree of consumption by attrition is between 2 and 8 % and its stability corresponds to a weight loss of active oxygen of between 20 and 55 % after 50 days of storage in ambient air at 55 _° C.

12. Sodium perborate according to Claim 10 or 11, characterised in that its specific surface area is between 4 and 8 m²/g.

13. Washing powders for textiles, containing sodium perborate monohydrate according to any one of Claims 10 to 12.